git doc: direct bug reporters to mailing list archive

git doc: direct bug reporters to mailing list archive

The mailing list archive can help a user encountering a bug to tell
whether a recent regression has already been reported and whether a
longstanding bug has already had some discussion to start their
thinking.

Based-on-patch-by: Martin Ågren <martin.agren@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Improved-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>